DESCRIPTION (from applicant's abstract) Nitric oxide (NO) has been recognized as a ubiquitous intercellular messenger in physiological systems involved in vasodilatation, neurotransmission, and cytotoxicity. The particular processes, physiological or pathological, and the molecular mechanisms by which NO acts may be largely determined by local conditions, e.g., the local redox milieu and by the concentrations, time course and spatial relation of NO available relative to those potential reactant species e.g., oxygen, superoxide, heme- and metal-containing proteins and thiols. Despite intensive investigation, the molecular mechanisms by which NO exerts its physiological effects are still obscure. This conundrum is most likely due to several factors: 1) widespread reliance on NO donor compounds, which have complex pharmacokinetics and carrier species which exert unintended physiological effects; 2) indirect methods, e.g., bioassay, measurement of NO metabolites and measurement of NO target activity and 3) the use of tissue preparations e.g., cell culture, synaptosomes having conditions different than those of the physiological system. The proposed studies intend to circumvent these ambiguities by direct measurement of extracellular concentration changes of both endogenous glutamate and NO simultaneously in real time using electrochemical microsensors in the anesthetized rat in vivo. The hypothesis that NO acts as an intercellular messenger at the perforant path-granule cell synapse of the dentate gyrus where it modulates release of glutamate and the molecular mechanisms involved will be investigated. Synaptic overflow of endogenous glutamate elicited by electrical stimulation of the afferent glutamatergic pathway will be used to quantitate changes in release and uptake of glutamate using a mathematical model and correlate them with changes in extracellular NO. Proposed in vivo measurements of NO diffusion should yield novel information about the magnitude of physiological changes in NO concentration and the spatial and temporal sphere of NO influence.